Fixed layout of blog test.

- Added main tags and replaced div with section.
- Added banner component to blog posts.

src/routes/blog/[slug].svelte

@@ -14,6 +14,7 @@
 </script>
 
 <script>
+    import Banner from '../../components/banner.svelte'
     export let post;
 </script>
 
@@ -57,8 +58,11 @@
     <title>{post.title}</title>
 </svelte:head>
 
-<h1>{post.title}</h1>
+<Banner/>
 
-<div class='content'>
+<main class="container">
+    <h1>{post.title}</h1>
+    <section>
         {@html post.html}
-</div>
+    </section>
+</main>